It is well known in the metal finishing art that metal surfaces such as aluminum, iron, steel, galvanized and zinc surfaces may be coated with an inorganic phosphate by contacting the surface with an aqueous phosphating solution. The phosphate coating protects the metal surface to a limited extent against corrosion and serves primarily as an excellent base for the later application of corrosion-inhibiting compositions and siccative organic coating compositions such as oils, waxes, paint, laquer, varnish, primers, synthetic resins, enamel, and the like.
The inorganic phosphate coatings generally are formed on a metal surface by means of aqueous solutions which contain phosphate ion, and optionally, certain auxiliary ions including metallic ions such as sodium, manganese, zinc, cadmium, copper, lead, and antimony ions. These aqueous solutions also may contain non-metallic ions such as ammonium, chloride, bromide, fluoride, nitrate, sulfate, and borate ions. These auxiliary ions influence the reaction with the metal surface, modify the character of the phosphate coating and adapt it for a wide variety of applications. Other auxiliary agents such as oxidizing agents, coloring agents and metal cleaning agents also may be incorporated in the phosphating solution.
Such phosphating solutions are well known in the art and are effective in improving the adhesion of paint to metal surfaces. Although the adhesion of the siccative organic coating to the metal surfaces is improved by the phosphate coating, it has been noted, for example, where ferrous metal, galvanized ferrous metal or phosphated ferrous metal parts are provided with a siccative top coat of laquer or enamel, and such top coat is scratched or scored during, for example, handling, forming or assembling operations, the metal substrate becomes a focal point for corrosion and for a phenomenon known as “undercutting”. Undercutting, or the loosening of the top-coat and areas adjacent to a scratch or score causes a progressive flaking of the top-coat from the affected area. The undercutting also results in a reduction of the desirable corrosion-resistance properties.
In addition, phosphating solutions are necessarily highly acidic and thus require special handling and appropriate equipment. Sludge formation in the phosphating baths also can be problematic, and spent phosphating solutions and rinse waters require treatment prior to disposal to meet stringent state and local regulations pertaining to phosphate in effluent streams.